The operation of liquid crystal devices (e.g. liquid crystal displays and light valves and liquid crystal polymer elements such as optical retarders, polarisers, cholesteric filters etc.) requires controlled alignment and usually also pretilt of the liquid crystals. Currently, a mechanical rubbing technique is used to prepare surfaces which are capable of inducing alignment and pretilt.
To overcome the disadvantages of the rubbing technique, several optical methods have been developed, which use linearly polarised light and are generally called photo-alignment methods. These are disclosed in U.S. Pat. Nos. 4,974,941 Gibbons et al, 5,784,139 Chigrinov et al, 5,389,698 Chigrinov et al, and European Patent 0525478B (Hoffman-La Roche et al).
While satisfactory in themselves, the methods disclosed in these patents rely on polarised light. Light sources producing polarised light are comparatively complex, which may be less suitable for mass production, and expensive. As a polariser usually absorbs at least 50% of the light, dispensing with the polariser would allow a much better usage of the light source (either a faster effect, or could use a weaker lamp). Therefore, already certain methods have been suggested, which use non-polarised light.
The generation of a pretilt angle in a nematic liquid crystal cell using a polyimide surface which has been irradiated with non-polarised U.V. light with an angle of incidence of 70° to the normal to the surface has been disclosed by Seo et al in “Asia Display 98” paper P-81, pp 795-798 and in “Liquid Crystals”, 1997 vol 23 no. 6 pp 923-925. However, this method does not benefit from the potential advantages which we have identified, requiring instead very high energy input, sufficient to depolymerise the polyimide.
We have found that under certain unexpected circumstances the optical photo-alignment method referred to above also works with light that is non-linearly polarised (e.g. circularly polarised) or isotropic (unpolarised).